Expose, Spotlight Search, The Dock, Toast Popups, The fact you can interact with another window that isn't your currently active window (Close it, Minimize, Scroll in it etc).
The quality of the 3rd party Apps. Movist for video is way better than VLC in interface and responsiveness as one example. I also really like Apples Mail program. iMessage is excellent on OS X. There are lots of really well made OS X applications like these from 3rd parties. Transmit, TextWrangler, 1Password, TweetDeck, Adium.
To give OS X a fair shake you really need to use it for a month and put all your normal work in to it. Setup the email, setup your skype or IM, use the FTP programs and the text editors, try iPhoto or Aperture etc
Personally I prefer it to Windows but I don't game on OS X. To me it's a place to get work done very efficiently and I also use it as my life machine with regards to browsing and email and IMing. The only thing I use Windows for is Gaming and Programming at this point.